The objective of our research proposal is to delineate the mechanisms of action of posterior pituitary hormones, in particular oxytocin and antidiuretic hormone (ADH), on renal excretion of sodium and chloride in adult and newborn dogs during volume expansion. Specifically, experiments are designed to determine whether: 1) serum oxytocin and/or ADH concentrations correlate with the degree of volume expansion and resultant Na excretion rates, 2) there is a maturational relationship between serum oxytocin or ADH levels and Na excretion, 3) vagotomy reduces the saline expansion-induced oxytocin and/or ADH release and whether these effects can be reversed by electrical stimulation of the central vagi stumps, 4) the natriuretic effects of oxytocin and ADH are a consequence of inhibition of sodium reabsorption in the proximal or distal segments of the tubule, and 5) changes in calcium permeability are involved in the natriuretic action of ADH.